The present invention relates to a device for dewatering sewage sludge to produce a moist friable solid substantially free of excess water and suitable for disposal without further processing and with minimal handling. The present invention further involves a method for dewatering sewage material in a single device which can be fixed or portable wherein the portable unit can optionally be utilized for both dewatering sludge and transporting the dewatered sludge to a disposal site without intermediate handling of the dewatered sludge.
Sewage sludge is a gelatinous, thixotropic semi-liquid mass removed from the liquid flow of sewage that, depending on its water content, typically behaves as a non-Newtonian fluid. The sludge will vary in amount and characteristics from different sources and with varying plant operation. Its physical properties make the pumping, dewatering, and/or handling of sludge by conventional means difficult and complicate its transportation and disposal. Sludge from primary treatment is composed of solids which can typically have a moisture content ranging from about 75 to about 95 percent. The physical handling and disposal of sewage sludge would be simplified if the solids could be more readily separated and if greater amounts of water could be readily removed. However, dewatering of sludge has proven difficult due to the nature of the substance. Dewatering sludge by conventional means, such as filtration and/or normal evaporation, is impractical because of its colloidal nature and because the water is entrapped by the molecular structure of the sludge causing the sludge to form a crust which further inhibits air drying.
The disposal of sludge presents issues related to both environmental considerations and energy consumption. Currently both land and water disposal of unprocessed sludge are widely practiced, but it is generally recognized that these methods cannot continue indefinitely. Disposal by burning has proven inefficient because today's dewatered sludge typically contains large amounts of residual water. Incineration requires enormous quantities of energy to drive off the trapped water contained by the sludge before combustion can occur. Land disposal is problematic because of the tendency of today's dewatered sludge to slowly give off (weep) additional contaminated water (excess water) once deposited at a landfill that can enter surface and ground water sources near the disposal site.
Efforts to efficiently dewater sludge by mechanical means such as vacuum filters, belt presses, centrifuges, and other mechanical devices have provided limited success. The removal of water has proven slow and inefficient. In addition, these processes require substantial capital investment and are costly to operate. The recovered, mechanically dewatered, sludge can be heat-dried, if it is to be reclaimed, but because the solids still contain large amounts of water, these further processes are also energy intensive and costly. In large communities where substantial amounts of sludge are produced and where the necessary capital costs can be supported, mechanical dewatering and incineration are more often practiced. In smaller and intermediate size communities landfill disposal and composting are practiced. Smaller communities, particularly some rural areas, often rely on the sand bed method to dewater sludge. This process is carried out by spreading the sludge over an expansive bed of sand, allowing excess water to slowly filter through the sand and/or evaporate, and after an extended period of time physically collecting the dewatered sludge along with the top layer of sand for disposal. This dewatering method is time and space intensive and has limited utility.
Each of the methods described above can be marginally improved by adding a flocculant to sludge prior to dewatering. The flocculant facilitates the build-up of colloidal materials and other small suspended particles into larger particles which are more readily separated from the sludge's water phase. Common flocculants include inorganic flocculants [e.g. poly (iron sulfate)] and polymeric anionic, cationic, nonionic, or amphoteric flocculants. Even with flocculation of the sewage sludge, equipment and handling costs are still substantial and obtaining dewatered sludge with low water content remains problematic.
What is needed is a simple, efficient, and inexpensive method for dewatering sludge to provide a friable solid free of excess water which can be efficiently incinerated without further drying, directly disposed of in landfills, and is suitable for directly composting without further treatment. Similarly, a device capable of carrying out this improved method is needed. The process utilizing this new device should minimize handling of the sludge and dewatered material and be mobile or capable of installation and operation in any size facility without excessive capital investment. The present disclosure addresses these needs and provides a novel device for conducting the disclosed method.